This invention is directed to conduits, and more particularly to conduits which have exceptional mechanical integrity and which are adaptable for use in carrying fluids or motion transmitting members.
Conduits for transporting fuel and/or fuel vapors to and from the engines of cars, buses, boats, airplanes and the like are important components of the fuel delivery systems of such vehicles. These conduits must possess a variety of characteristics and features, not the least of which are resistance to vapor permeability and chemical degradation. The need for these features has recently become acute due to increasingly stringent environmental regulations and to the increased emphasis on the development and use of oxygen containing compounds in fuels and fuel additives. Many of these compounds tend to severely degrade the materials heretofore used in fuel system conduits.
One material which possesses exceptional resistance to chemical degradation and vapor permeation is polytetrafluoroethylene (PTFE). Accordingly, such materials are desirable for use as liners in conduits used in fuel delivery systems. However, PTFE and like materials also frequently possess characteristics which make their use in such conduits difficult. For example, fluorocarbon polymers are well known for their "slipperiness," which sometimes makes the construction of a mechanically stable conduit difficult, thereby limiting their application.
U.S. Pat. No. 3,050,786--St. John, et al. relates to conduits lined with PTFE. While this patent recognizes the advantageous features of such fluorocarbon polymers, it also recognizes their deficiencies:
Although (PTFE) has considerable toughness and can withstand temperatures in normal usage upwards of 200.degree. F., it is still a plastic without mechanical strength available in metals or other non-plastic materials. In view of the above, a need has developed for a satisfactory method of combining (PTFE) with another material having the necessary mechanical strength. PA1 If one were seeking a pipe, for example, having the strength of steel and the chemical inertness of PTFE, the obvious solution would appear to be to line a steel pipe with the resin. Unfortunately, this is more easily said than done. A lining in order to be acceptable must be provided in such a manner that separation from the pipe wall is precluded during the anticipated life of the pipe. As a lining, the material must have sufficient thickness so as to render it impermeable. Considerable difficulty has been encountered in attempting to employ dispersions of PTFE for coating the interior of a pipe. Multiple coats must be applied in order to build up sufficient wall thickness. At the same time, however, a satisfactory method has not been found for perfecting a bond between such a coating and other materials. The alternative method of forcing an outsized liner into a pipe bore cannot be utilized for lengths in excess of a few inches.
Thus, there has been a long felt need for conduits which incorporate a mechanically stable fluorocarbon inner liner or coating. As explained in the patent to St. John, et al.:
The patent to St. John, et al. attempts to overcome the above-noted difficulties by providing a tube of prestressed, sintered PTFE resin which is dimensionally stable at room temperature but which alters its girth upon heating. In this way, the PTFE tube can be inserted into the outer sheath of the conduit and then heated so as to expand into tight conformity therewith.
While the solution presented by St. John, et al. may prove advantageous in certain applications, applicants have found that such techniques can also have disadvantages. For example, the outer casing to which the PTFE is joined may not itself possess thermal stability at the temperatures required to expand and/or shrink the PTFE resin into conformity therewith. Additionally and importantly, applicants have found that tubular PTFE products, especially thin walled tubular PTFE products, are highly susceptible to kinking and deformation during processing thereof. Such kinking and/or deformation is highly detrimental since it tends to substantially reduce the resistance of the PTFE material to vapor permeation. For this additional reason, therefore, methods of the type disclosed by the St. John et al. patent may be unacceptable due to deformation or kinking the PTFE liner.
Applicants have discovered that it is possible to form a strong, mechanically stable, and highly vapor permeation resistant tubular composite comprising an outer plastic jacket and an inner PTFE liner without the use of either adhesive bonding or macroscopic mechanical joining techniques. The prior art has recognized the extreme difficulty associated with joining fluoropolymer articles to other materials, even if adhesive bonding is involved. See, for example, U.S. Pat. No. 4,744,857--Nelson, wherein it is noted that numerous difficulties are associated with the use of chemical etching techniques to aid in adhesive bonding of fluorocarbon polymers.
The process of chemical etching typically includes introducing the surface material to be treated into a powerful etchant, such as a treatment solution of an alkali metal in an amine-type solvent (including liquid ammonia). Such a process, which is disclosed in U.S. Pat. No. 2,789,063--Purvis, et al, is known to be an extremely harsh procedure, and the disadvantages of this approach are that the surface degrades easily and there are waste disposal and pollution problems. Another basic disadvantage of this method is in the inconvenience, difficulty and hazard of handling a solvent such as liquid ammonia. Moreover, these techniques are known to not be applicable to all types and formulations of fluorocarbon polymer articles. Because of all these difficulties and for other reasons, tubular composites formed from adhesive-free, chemically etched fluorocarbon polymer liner have not to applicants knowledge been heretofore formed.
Accordingly, it is an object of the present invention to provide conduits having mechanical integrity and resistance to chemical degradation.
It is also an object of this invention to provide fuel system conduits having both mechanical integrity and resistance to vapor permeation.
It is a further object of the present invention to provide fuel system conduits and wire carrying conduits having liners comprising fluorocarbon polymer.
It is yet another object of the present invention to provide methods for forming tubular conduits in which the inner plastic liner thereof is not subject to deleterious kinking or deformation during the manufacture thereof.
It is an even further object of the present invention to provide a harmless, satisfactory method for perfecting a bond between a fluorocarbon polymer liner and a polymeric jacket.